cassandra-user mailing list archives

Did you mean to ask "are 'all' your keys spread across all SSTables"? I am guessing at your
intention.
I mean I would very well hope my keys are spread across all sstables or otherwise that sstable
should not be there as he has no keys in it ;).
And I know we had HUGE disk size from the duplication in our sstables on size-tiered compaction….we
never ran a major compaction but after we switched to LCS, we went from 300G to some 120G
or something like that which was nice. We only have 300 data point posts / second so not
an extreme write load on 6 nodes as well though these posts causes read to check authorization
and such of our system.
Dean
From: Kanwar Sangha <kanwar@mavenir.com<mailto:kanwar@mavenir.com>>
Reply-To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>
Date: Friday, March 22, 2013 8:38 AM
To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>
Subject: RE: High disk I/O during reads
Are your Keys spread across all SSTables ? That will cause every sstable read which will increase
the I/O.
What compaction are you using ?
From: zodiak@fifth-aeon.net<mailto:zodiak@fifth-aeon.net> [mailto:zodiak@fifth-aeon.net]
On Behalf Of Jon Scarborough
Sent: 21 March 2013 23:00
To: user@cassandra.apache.org<mailto:user@cassandra.apache.org>
Subject: High disk I/O during reads
Hello,
We've had a 5-node C* cluster (version 1.1.0) running for several months. Up until now we've
mostly been writing data, but now we're starting to service more read traffic. We're seeing
far more disk I/O to service these reads than I would have anticipated.
The CF being queried consists of chat messages. Each row represents a conversation between
two people. Each column represents a message. The column key is composite, consisting of
the message date and a few other bits of information. The CF is using compression.
The query is looking for a maximum of 50 messages between two dates, in reverse order. Usually
the two dates used as endpoints are 30 days ago and the current time. The query in Astyanax
looks like this:
ColumnList<ConversationTextMessageKey> result = keyspace.prepareQuery(CF_CONVERSATION_TEXT_MESSAGE)
.setConsistencyLevel(ConsistencyLevel.CL_QUORUM)
.getKey(conversationKey)
.withColumnRange(
textMessageSerializer.makeEndpoint(endDate, Equality.LESS_THAN).toBytes(),
textMessageSerializer.makeEndpoint(startDate, Equality.GREATER_THAN_EQUALS).toBytes(),
true,
maxMessages)
.execute()
.getResult();
We're currently servicing around 30 of these queries per second.
Here's what the cfstats for the CF look like:
Column Family: conversation_text_message
SSTable count: 15
Space used (live): 211762982685
Space used (total): 211762982685
Number of Keys (estimate): 330118528
Memtable Columns Count: 68063
Memtable Data Size: 53093938
Memtable Switch Count: 9743
Read Count: 4313344
Read Latency: 118.831 ms.
Write Count: 817876950
Write Latency: 0.023 ms.
Pending Tasks: 0
Bloom Filter False Postives: 6055
Bloom Filter False Ratio: 0.00260
Bloom Filter Space Used: 686266048
Compacted row minimum size: 87
Compacted row maximum size: 14530764
Compacted row mean size: 1186
On the C* nodes, iostat output like this is typical, and can spike to be much worse:
avg-cpu: %user %nice %system %iowait %steal %idle
1.91 0.00 2.08 30.66 0.50 64.84
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
xvdap1 0.13 0.00 1.07 0 16
xvdb 474.20 13524.53 25.33 202868 380
xvdc 469.87 13455.73 30.40 201836 456
md0 972.13 26980.27 55.73 404704 836
Any thoughts on what could be causing read I/O to the disk from these queries?
Much thanks!
-Jon